In a water-cooled nuclear reactor that has operated for a significant length of time, radioactive fission products build up in the reactor core. These fission products generate radioactive heat even when the reactor is shut down, with typical decay heat generation being one percent of the reactor full-power heat generation rate. If the reactor core is not cooled after shutdown, either by use of normal shutdown procedures or post-accident shutdown procedures, the reactor core may melt. Depending on details of reactor design and mode of reactor shutdown, decay heat removal may be accomplished by cooling the hot water reactor or allowing the reactor water to boil with steam exiting the reactor and adding makeup water to the reactor. In either case, active cooling systems are used to cool the reactor and prevent a reactor core meltdown.
Active systems such as these can fail due to equipment failure or operator error as happened at Three Mile Island. What is needed is a passive decay heat removal system of high reliability that becomes automatically activated upon loss of coolant and which may have its operability verified during normal operations.
Various approaches toward devising passive cooling systems for water-cooled reactors are disclosed in the following papers presented at the International Atomic Energy Agency Technical Committee Meeting on Passive Safety Features in Current and Future Water-Cooled Reactors in Moscow on Mar. 21-24, 1989:
Application of Passive Systems in WWER-1000 Design Project of Increased Safety: Part I, V. I. Naletov, G. A. Tarakov, E. M. Damrin, and N. B. Trunov. PA1 Application of Passive Systems in WWER-1000 Design Project of Increased Safety: Part II, T. A. Brantova and N. S. Fil. PA1 Analysis of the Possibility to Increase WWER-440 Safety Level on the Base of Passive Systems, B. Dimitrov.
The systems disclosed in these papers rely on opening of air doors to obtain cooling or depend upon flooding of the reactor by water located above the reactor containment level. Placement of an open-top box inside the pressure vessel in combination with a closed circuit heat exchanging loop as in the present invention is not disclosed in the prior art known to the applicant.